89Zr-trastuzumab PET visualises HER2 downregulation by the HSP90 inhibitor NVP-AUY922 in a human tumour xenograft

NVP-AUY922, a potent heat shock protein (HSP) 90 inhibitor, downregulates the expression of many oncogenic proteins, including the human epidermal growth factor receptor-2 (HER2). Because HER2 downregulation is a potential biomarker for early response to HSP90-targeted therapies, we used the 89Zr-labelled HER2 antibody trastuzumab to quantify the alterations in HER2 expression after NVP-AUY922 treatment with HER2 positron emission tomography (PET) imaging.The HER2 overexpressing human SKOV-3 ovarian tumour cell line was used for in vitro experiments and as xenograft model in nude athymic mice. In vitro HER2 membrane expression was assessed by flow cytometry and a radio-immuno assay with 89Zr-trastuzumab. For in vivo evaluation, mice received 50 mg/kg NVP-AUY922 intraperitoneally every other day. 89Zr-trastuzumab was injected intravenously 6 d before NVP-AUY922 treatment and after 3 NVP-AUY922 doses. MicroPET imaging was performed at 24, 72 and 144 h post tracer injection followed by ex-vivo biodistribution and immunohistochemical staining.After 24 h NVP-AUY922 treatment HER2 membrane expression showed profound reduction with flow cytometry (80%) and radio-immuno assay (75%). PET tumour quantification, showed a mean reduction of 41% (p = 0.0001) in 89Zr-trastuzumab uptake at 144 h post tracer injection after NVP-AUY922 treatment. PET results were confirmed by ex-vivo 89Zr-trastuzumab biodistribution and HER2 immunohistochemical staining.NVP-AUY922 effectively downregulates HER2, which can be monitored and quantified in vivo non-invasively with 89Zr-trastuzumab PET. This technique is currently under clinical evaluation and might serve as an early biomarker for HSP90 inhibition in HER2 positive metastatic breast cancer patients.